1. Field of the Invention
The present invention relates to a process for producing sticky polymers and more particularly to a process for producing sticky polymers in gas phase reactors at reaction temperatures in excess of the softening temperature of said sticky polymers.
2. Description of the Prior Art
The introduction of high activity Ziegler-Natta catalyst systems has lead to the development of new polymerization processes based on gas phase reactors such as disclosed in U.S. Pat. No. 4,482,687 issued Nov. 13, 1984. These processes offer many advantages over bulk monomer slurry processes or solvent processes. They are more economical and inherently safer in that they eliminate the need to handle and recover large quantities of solvent while advantageously providing low pressure process operation.
The versatility of the gas phase fluid bed reactor has contributed to its rapid acceptance. Alpha olefins polymers produced in this type of reactor cover a wide range of density, molecular weight distribution and melt indexes. In fact new and better products have been synthesized in gas phase reactors because of the flexibility and adaptability of the gas phase reactor to a large spectrum of operating conditions.
The term "sticky polymer" is defined as a polymer, which, although particulate at temperatures below the sticking or softening temperature, agglomerates at temperatures above the sticking or softening temperature. The term "sticking temperature", which, in the context of this specification, concerns the sticking temperature of particles of polymer in a fluidized bed, is defined as the temperature at which fluidization ceases due to excessive agglomeration of particles in the bed. The agglomeration may be spontaneous or occur on short periods of settling.
A polymer may be inherently sticky due to its chemical or mechanical properties or pass through a sticky phase during the production cycle. Sticky polymers are also referred to as non free flowing polymers because of their tendency to compact into agglomerates of much larger size than the original particles. Polymers of this type show acceptable fluidity in a gas phase fluidized bed reactor; however, once motion ceases, the additional mechanical force provided by the fluidizing gas passing through the distributor plate is insufficient to break up the agglomerates which form and the bed will not refluidize. These polymers are classified as those, which have a minimum bin opening for free flow at zero storage time of two feet and a minimum bin opening for free flow at storage times of greater than five minutes of 4 to 8 feet or more.
Sticky polymers can also be defined by their bulk flow properties. This is called the Flow Function. On a scale of zero to infinity, the Flow Function of free flowing materials such as dry sand is infinite. The Flow Function of free flowing polymers is about 4 to 10, while the Flow Function of non-free flowing or sticky polymers is about 1 to 3.
Although many variables influence the degree of stickiness of the resin, it is predominantly governed by the temperature and the crystallinity of the resin. Higher temperatures of the resin increase its stickiness while less crystalline products such as very low density polyethylene (VLDPE), ethylene/propylene monomer (EPM), ethylene/propylene diene monomer (EPDM) and polypropylene (PP) copolymers usually display a larger tendency to agglomerate to form larger particles.
Thus the prior art has heretofore attempted to produce polymers at temperatures below the softening temperature of the polymers. This is based primarily on the fact that operating at or above the softening temperature would cause serious agglomeration problems. Indeed BP Chemicals Limited, PCT International Publication Number WO 88/02379, published April, 1988 which discloses use of 0.005% to less than 0.2% of a pulverulent inorganic substance during the reaction, nevertheless cautions against the use of temperatures in excess of the softening temperatures of the produced polyolefin. Moreover, this reference specifically discourages use of quantities of pulverulent inorganic substances in the reactor in excess of 0.2% by weight since as stated therein there is no further improvement in the polymerization or copolymerization process in the gaseous phase and use of amounts in excess of 0.2% deleteriously affect the quality of the polymer or copolymer produced.
It would be extremely beneficial to conduct these type polymerizations at temperatures at or above the softening point of the sticky polymers since it is well known that increases in polymerization temperatures generally enhance the yield of product in relation to the catalyst. In addition, purging of the polymer product becomes more efficient.